1. Field of the Invention
The present invention relates to a technology to detect flicker and correct the detected flicker. More particularly, the present invention relates to an image-data processing apparatus and an image pickup apparatus that detect flicker occurring in a video image captured with illuminating light the brightness of which varies with the power frequency to correct the detected flicker.
2. Description of the Related Art
Although the problem of flicker is frequently approached in image pickup devices having a rolling shutter function, blinking over the entire screen occurs in image pickup devices having a global shutter function. Such blinking is called plane flicker. The plane flicker is caused in a case in which the blinking cycle (for example, the blinking cycle of the illuminating light is equal to 1/120 seconds in West Japan and is equal to 1/100 seconds in East Japan) of the illuminating light is not an integral multiple of the imaging cycle. The blinking cycle of the illuminating light is determined by the power frequency. For example, in Japan, since the power frequency in West Japan is set to 60 Hz, the blinking cycle of the illuminating light in West Japan is equal to 1/120 seconds. In contrast, since the power frequency in East Japan is set to 50 Hz, the blinking cycle of the illuminating light in East Japan is equal to 1/100 seconds.
It is a common knowledge that the plane flicker occurs at a frequency that is equal to the least common multiple of the imaging frequency of an image pickup apparatus and the blinking frequency of the illuminating light. In order to remove the plane flicker having the above characteristics, the correction gain to be applied to digital signals (image data) captured by the image pickup devices having the global shutter function is varied in related art.
A method of determining the correction gain in the related art will now be described with reference to FIGS. 8(a) and 8(b).
FIGS. 8(a) and 8(b) illustrate a technology in the related art to correct the plane flicker when a small change occurs in a target of the image capturing.
FIG. 8(a) illustrates a waveform indicating the relationship between the imaging frequency and the blinking frequency of the illuminating light.
Referring to FIG. 8(a), the vertical axis represents the strength of the illuminating light and the horizontal axis represents time.
FIG. 8(b) illustrates a waveform indicating the relationship between a digital signal generated by photoelectric conversion in an image pickup device and the correction gain based on the digital signal.
Referring to FIG. 8(b), the vertical axis in the upper waveform represents the level of the digital signal, that is, the brightness of each frame. The vertical axis in the lower waveform represents the magnitude of the correction gain. The horizontal axis representing time is common to the upper and lower waveforms.
For example, when the imaging cycle, that is, the exposure time is equal to 1/180 seconds in an image pickup apparatus, the imaging frequency of the image pickup apparatus is equal to 180 Hz. In the image capturing with the illuminating light having a blinking frequency of 120 Hz by the image pickup apparatus, the cycle of the plane flicker appearing in moving images captured by the image pickup apparatus is equal to 60 Hz, which is the greatest common measure of the imaging frequency and the blinking frequency. In this case, the sum (hereinafter referred to as a “total pixel level”) of the pixel levels in each frame of the digital signal output from the image pickup device is varied at a frequency of 60 Hz, as shown in the upper waveform in FIG. 8(b).
In the related art, the correction gain is determined by using data items in phase with each other immediately before the frame to be corrected according to a fact that the least common multiple of the imaging frequency and the blinking frequency of the illuminating light is equal to the flicker frequency. As shown in the lower waveform in FIG. 8(b), the effective correction can be achieved by this method when a small change occurs in the target of the image capturing. For example, a technology to determine correction gains against the flicker of this type is disclosed in Japanese Unexamined Patent Application Publication No. 2001-111887.